Means for digitally indicating fractions of an analog signal



March 20, 1962 D L. SPOONER Filed May 19, 1958 TENS 3,026,033 MEANS FOR DIGITALLY INDICATING FUNCTIONS OF AN ANALOG SIGNAL 4 Sheets-Sheet l l l I I DAVID L. SPOONER March 20, 1962 D. L. SPOONER 3,026,033

MEANS FOR DIGITALLY INDICATING FUNCTIONS OF AN ANALOG SIGNAL INVENTOR DAVID L. SPOONER March 20, 1962 Filed May 19, 1958 D. L. SPOONER MEANS FOR DIGITALLY INDICATING I FUNCTIONS OF AN ANALOG SIGNAL 4 Sheets-Sheet 3 VOLTAGE CONTROLLED 2 OSCILLATOR is 86 5 O 66 mom: RING I LSE MODULATOR- X DEMODULATOR D HUNDREDS 24 202 i 'u l2b\ J T Y 50 PULSE C AMP 7 A SUMMING MODULATOR- NETWORK DEMODULATOR I60 rzvygp rofl i, i, 206 DAVID L. SPOONEF 4 Sheets-Sheet 4 I/VVENTOI? DAVID L. SPOONER wwm March 20, 1962 D. SPOONER MEANS FOR DIGITALLY INDICATING FUNCTIONS OF AN ANALOG SIGNAL Filed May 19, 1958 M4 9m mu fi E E J 3 E WW MEQSEZ W W OmN mm N m% fi vm :55 Nmm :05 W M 9 ww vw jmvwo 0% 3w mzwk NNN sum L Om |||Iv 00 OWN United States Fatent 3,026,033 MEANS FOR DIGIT LLY INDECATING FRAC- TIONS OF AN ANALOG SIGNAL David L. Spooncr, Columbus, Ohio, assignor to Industrial Nucleonics Corporation, a corporation of Ohio Filed May 19, 1958, Ser. No. 736,221 8 Claims. (Cl. 235-454) This invention relates to a digitizer, and more particularly it relates to a novel system for converting an analog signal to a digital indication.

In automatic data processing systems, there are innumerable uses for devices of this type, which may constitute an interconnecting link between continuous, intelligencebearing electrical signals and digital computers, recorders and/or controllers for processing and utilizing said signals. The present system is in a class of medium-speed digitizers which may be employed in periodically sampling one or a plurality of condition-rmponsive electrical voltages, and in which use is capable of rendering on the order of ten to one hundred conversions per second.

A large number and a great variety of digitizers in this class are presently known to those familiar with the art. It would therefore be inappropriate in this specification to present a discussion of the relative merits of each and the advantages thereover to be gained through the use of the instant device. However, as is known also, at least the majority of these systems suffer from one or more shortcomings; among which can be listed a lack of high accuracy and reproducibility, extreme complexity at the expense of reliability, and/or a prohibitively high cost for many installations which could otherwise be improved to an appreciable extent by the ready availability of digital data.

The present invention employs the voltage comparison method, utilizing a source of constant reference potential applied through a resistancepotentiometer type voltage divider, in the manner of mechanical switch-operated digitizeis which are characterized by the high accuracy and reproducibility. In the interests of speed, there is provided novel circuitry whereby the function of the mechanical switch is performed by an ionic and/or electronic beam switching device, preferably of the decade distributor tube variety. Since the beam switching device is in itself a digital counter, the digital conversion is obtained directly, and in convenient decimal form. In connection with the performance of its switching function, the counter itself must generate the reference potential at each step in the conversion; the constant voltage of the nominal reference source being utilized to clamp the counter-generated reference voltage at the correct value. Accordingly, it is not possible for the counter indication to get out of step with the generatedreference potential. The switching pulses which advance the counter-generator are initiated by an oscillatordriven comparator-modulator responsive to the polarity of a DC. signal produced by algebraically adding the signal voltage to the counter-generated reference voltage.

In one preferred form of the invention, the reference voltage initially increases as a regular stairstep function paced by the frequency of the driving oscillator. The comparator-modulator accurately detects the crossover point where the signal voltage is equalled or exceeded by the reference voltage, and thereupon ceases to provide the pulses which advance the counter. In order to make full use of the inherent counting speed of the beam distributor tubes, the oscillatormay be designed to operate at -a frequency where the reference voltage to the comparator is subject to an appreciable lag as a, result of filter and stray capacitances in the circuit.

To prevent the counter-generator from overshooting 3,@Z6,ll33 Patented Mar. 20, 19632.

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7 ing pulses to the counter while the added velocity component of the reference voltage decays across the lead network. The result obtained is a high counter switching rate while a substantial difference between the reference and signal voltages obtains, and a switching rate which approaches zero near the point of balance of the two signals, even though the driving oscillator frequency is unchanged. In an alternate embodiment, the algebraic sum of the signal and reference voltages may be applied to a frequency-determining reactance tube circuit or other circuit whereby the oscillator frequency is automatically varied as a function of the amount of the unbalance.

In accordance with a still further novel feature of the invention, there is provided a digitizer employing circuitry which may be termed a digital follower. With this system, it is not required that the counter start at zero in order to effect each digital conversion. Through the use of a bidirectional counting device oriented by a dual-output comparator-modulator, the system is able to start each conversion with the reference voltage value standing in the register as the result of the previous conversion; either adding or sulbstracting digits until balance again is obtained. The digital follower is particularly efficient in applications where it is desired to obtain a digital record of the short-term variations in a continuous analog signal. In such applications the digitizer may provide a very rapid transcription of the analog signal in a shorthand consisting merely of add and subtract pulses rather than complete digital numbers.

As will be apparent in view of the circuit description which follows, the present digitizer not only can be made to indicate the input voltage values directly, but by the original choice of potentiometer resistors the device will as readily indicate the square, square root, logarithm or another predetermined function of the input voltage.

In the above described systems all counter-generated reference voltages are clamped to a standard, constant reference voltage source, as stated hereinabove. However, the potential utility of the present invention is multiplied by the fact that the reference voltages may as readily be clamped to a variable D.C. signal volt-age. Thusthe invention may be instantaneously converted to a system for digitizing the ratio of two voltages. This permits its use as a dividing element in a digital computer; accepting two analog signals v and providing a digital answer.

It is the principal object of the invention to provide a simple, accurate, reliable and inexpensive system for conventing analog signals to digital signals.

It is another object to provide a digitizer combining potentiometer accuracy with electronic speed.

It is still another object to provide a digitizer employing a counter-generated reference voltage function, whereby the register indication is always correlated with the value of said reference voltage.

It is yet another object to provide a digitizer in accordance with the preceding object wherein all values of said counter-generated voltage function are accurately determined by the value of a stable, constant-potential reference source.

It is a further object to provide a digitizer in accordance with the above objects, wherein the counter switch.-

ing operation is dependent entirely on an unbalance between the signal and reference voltages.

It is a still further object to provide a digitizer with a basically high counter switching rate, together with means for reducing the switching rate as the value of the reference voltage approaches the value of the signal voltage, whereby sufiicient time for an accurate comparison can be allowed without greatly reducing the digital conversion speed.

It is also an object to provide a digital follower for rapidly transcribing the variations in an analog signal.

It is an additional object to provide means for digitizing the value of a predetermined function of an analog signal.

Again it is an object to provide a digital computer element for directly digitizing the ratio of two voltages.

Other objects and advantages will become apparent in the following detailed description, taken in conjunction with the appended drawings, in which:

FIGURES 1a and 1b, when mutually connected by lines 10a and 12b, comprise a complete circuit diagram of'a digitizer in accordance with one form of the in vention.

FIGURE 2 shows a modification of the comparatorrnodulator section of FIGURE 1.

FIGURE 3 shows a modification of the hundreds digitizing network of FIGURE 1.

FIGURE4 shows a modification of a portion of FIG- URE 1.

FIGURE 5 is a circuit diagram of a digital follower in accordance with the invention.

FIGURE 6 illustrates the use of the invention as a digital computer element for digitizing the ratio of two signals.

Referring to FIGURES la and 1b, there is shown a digitizer having input terminals 10 and 12. Terminal I0 is adapted to receive a negative analog signal to be digitized, and 12 is the reference terminal therefor, which in the circuit shown is grounded. There is also shown a further grounded terminal 12a and a further input terminal 16 to which a suitable constant reference source or" potential, represented by the battery 18, is connected with polarity as shown.

Connected in cascade fashion are three cold-cathode i decade distributor tubes 20-24 which may be of the kind referred to as a Nomotron, which is manufactured by Standard Telephone and Cable Company of England, and which is distributed as type GIG-241E by Intellex Systems, Inc., New York, N.Y. Tubes 22 and 24 are identical with tube 20, which comprises a gas-filled envelope containing ten cathodes as at 26-30 and 32. Cathode 26 represents the zero cathode, 28 represents the one cathode, 3%) the two cathode, and 32 represents the nine cathode; the identical three to eight cathodes being omitted for simplicity in the drawing. Tube 20 further contains a glow discharge or beam switching electrode indicated at 34 and an extended anode 36. The anode 36 is connected to a positive plate voltage supply, represented by the battery 38, through an anode resistor 40 whose current limiting action aids in preventing the beam from forming on more than one cathode simultaneously.

It is seen that the beam switching electrode 34a of tube 22 is connected to the output of a pulse amplifier 42, whose input is connected through a coupling capacitor 44 to the zero cathode 26 of tube 20. A similar hook-up obtains between the switching electrode 34b of tube 24 and the zero 130 cathode of tube 22.

This

arrangement provides a rather conventional three-digit 4 48, and the tube 24' switches a hundreds digitizing network 50. The networks 46, 48 and 50, which are identical except for'the values of potentiometer resistors used therein, comprise one leg of an overall potentiometer network which also includes a lead network 52 and a summing network 54.

The lead network 52. comprises a resistor 56 and capacitor 58. Resistor 56 is in series connection with two summing resistors 60 and 62, forming a voltage divider connecting the digitizing networks terminating on line 64 with the common or ground line 12b.

The negative signal input to the digitizer on line 10a is applied to the junction of resistors 60 and 62 through a further summing resistor 66. Thejunction of resistors 56 and 60 exhibits a potential which is proportional to the sum of a digitized positive voltage generated in the counter-reference networks and the negative signal on line 10a. This sum is delivered via line 68 to a modulator unit 70.

In the illustrated preferred form, the modulator 70 includes an audio frequency oscillator and amplifier unit 72 which energizes the driving coil 74- of a chopper. The chopper incorporates a first vibrating switch contact 7411 which oscillates between a pair of stationary contacts 741'; and 740. Contact 74b is connected to the output of the summing network 54 and contact 740 is connected to the common line 12b. The vibrating contact 74a is connected through a coupling capacitor 76 to the input of an amplifier 73. The output of amplifier 78 is coupled through a transformer 80 and a blocking diode 82 to a demodulator switch section of chopper 74. The demodulator switch comprises a vibrating contact 74d which oscillates between a pair of stationary contacts 74e and 7-tf,' which latter contact is connected to the common line 12b as is one end of the secondary coil 80!; of transformer 8d. The other end of the transformer coil 80b is connected through the diode 82 to contact 74e.

In the operation of the modulator, the switch arm 74a is alternately connected to the sum of the reference and signal voltages and returned to ground potential. It said sum is not zero, an amplified alternating voltage will appear across the secondary 80b of output transformer 35). If the sum is a negative potential with respect to ground, the voltage across the secondary will be in a first phase relation to the vibration of the chopper switch arm 74d. If the sum is positive, the opposite phase relation obtains. The connections to the transformer are such that if the sum is negative the switch arm 74d will connect with a positive voltage on contact 74c, and if the sum is positive the voltage on contact 742 will be negative when the contact is made. The vibrating switch arm 74d is connected through a coupling capacitor 84 t0 the input of a pulse amplifier 86. The diode 82 will allow positive pulses to be transmitted to the input of the pulse amplifier, but substantially blocks the passage of negative pulses thereto. Even though the pulse amplifier is designed to respond to only one input polarity, the diode is appro priately used to prevent the usual differentiating action of the amplifier input circuits from triggering the amplifier on the rising slope of a negative pulse.

The output of the pulse amplifier 86 is connected via line 88 to the switching electrode 34 of the units counter tube 20, so that each pulse will advance the counter one unit in the conventional manner.

Referring now to the networks connected to the cathodes 2632 of the units counter tube 20, it is seen that the cathodes are individually connected to the common (mound) line 12b through cathode load resistors 1tl0- 106. The cathode resistors are respectively paralleled by' conventional time-constant capacitors 108-114. Each of? the cathodes with the exception of the zero cathode 26'. is also connected through a diode as at 11-6, 113 and to line 16a which is in turn connected to terminal 16 and the stable reference voltage source (battery) 18. Each. cathode, again with the exception of the zero cathode.

aoesoss Cathode Units Tens Hundreds Counter Counter Counter R R/lO R /100 R/2 R/2o 11/200 R/3 R/30 R/300 R/4 13/40 R/40O R/5 R/50 R/fiOO 11/6 11/60 R/600 R /7 R /70 R/700 R/8 R/80 R1800 R/9 R/90 R/QOO A somewhat oversimplified explanation of the operation of the circuits of FIGS. la and 1b is as follows. Assume that the counters have been reset to zero, and therefore the glow discharge or beam in each tube is formed on the zero cathode. A negative signal is now applied to line 10a, and the portion of this signal appearing across resistor 62 is connected through resistor 60 to contact 74b of the chopper. When the vibrating contact 74a contacts 74b, in the manner above described the pulse amplifier '86 delivers a pulse to the beam switching electrode 34 of tube 20, switching the beam or glow discharge from the zero cathode 26 to the one cathode 2 8 in known fashion. The beam current through cathode resistor 102 produces a voltage drop thereacross, raising the cathode 28 to a positive potential with respect to ground. When the cathode potential exceeds the potential of line 16a, however, any additional beam current is drawn from the reference voltage source 18 through the diode 116, thus clamping the cathode potential substantially at the value of the standard reference voltage. The stabilized cathode potential appears across the voltage divider combination of resistor 122, the lead network resistor 56- and resistors 60 and 62 in the summing network 54. Typically resistor 1 22 has a high value; e.g., fifty megoh-ms, whereas the remaining portion of the voltage divider has a relatively small impedance. Accordingly, a small portion of the cathode potential constitutes the positive reference potential which is summed with the signal voltage.

Assuming that upon the addition of the small increment (unit value) of positive reference voltage the chopper contact 74b is still negative due to the magnitude of the signal on line 10a, the next vibration of the chopper contact 74a will result in another pulse from pulse amplifier 86 being impressed onto the switching electrode 34 of tube 20. The glow discharge on cathode 28 is now transferred to the switching electrode and thence to the next cathode 30. As is known, the capacitor L10 across the cathode resistor 102 maintains the positive potential on cathode 28 only long enough to prevent the glowdischarge from returning thereto rather than forming on the other nearest cathode 30. As [the potential on cathode 2-8 is falling, the cathode 30 draws current through its cathode resistor 104, causing the cathode potential to rise above the potential of line 16a. Thereupon the cathode beam current is partially supplied from battery 18 through diode 118, clamping cathode 30 substantially at the potenial of battery 16. The stabilized cathode potential appears across the voltage divider combination of resistor 124- and the lead network and summing resistors. Since the value of resistor 1*2-4' is one-half the value of resistor 122, the portion of the cathode potential which appears in the summing network is twice as large as the unit value obtainedpreviously when the glow discharge was formed on cathode 28. Assuming that the addition of the double increment (two units) of reference potential leaves the chopper contact 74b still negative due to the magnitude of the signal on line 10a, another pulse from pulse amplifier 86 will advance the beam to the three cathode (not shown) of tube 24 The circuit of the three cathode is identical to that of the one and two cathodes 28 and 36, except that the associated potentiometer resistor has a value equal to oneathird of the resistance 122. Accordingly a triple increment (three units) of reference potential is presented to the summing network.

if the value of the signal input is sufficiently high, the counter will continue to'advance as described until the beam has been formed on the nine cathode 32. The next pulse from pulse amplifier 86 causes the beam to return to the zero cathode 26. The resulting potential rise thereof then transmits a pulse through capacitor 44, triggering pulse amplifier 42; which delivers an amplified pulse to the beam switching electrode 34a of the tens counter tube 22, thus causing the glow discharge therein to transfer from the zero cathode 138 to the one cathode 132. The value of the associated potentiometerresistor 134 is one-tenth of the value of resistor 122, so that when the cathode is clamped to the potential of the reference voltage source 18 the portion of the cathode voltage presented to the summing network is ten times the unit value.

Now if another pulse from the comparator-modulator advances the units counter one step further, so that the beam in tube 20 is formed on the one cathode 28; since both cathodes 28 and 132 are clamped at the potential of line 16a, the two potentiometer resistors 122 and 134 are effectively connected in parallel relationship to provide an impedance having a value equal to one-eleventh of the value of resistor 122. Accordingly eleven units of the reference potential are presented to the summing network.

The counting process and reference voltage build-up continues at the oscillator frequency until the reference voltage approaches the value of the signal voltage. It is understood that the designated reference voltage does not include the voltage drop across the lead network resistor 56. However, each increase in the potential on line 64- is momentarily by-passed around resistor 56 through capacitor 58. Thus a time derivative or velocity component of the designated reference voltage is added thereto. At some point in the counting process, the value of the designated reference voltage plus the derivative component will exceed the signal voltage, so that when chopper arm 74a returns to contact 74b the latter will be at a positive rather than a negative potential. Since the modulator 70 is unresponsive to this condition, the scheduled switching pulse does not occur. Mean-while the derivative component, comprising a charge on capacitor 58, decays so that on the next or one of the succeeding vibrations of the chopper arm another count and reference voltage increase may occur. The result of such pulse omissions is a gradual decrease in the count rate to zero as the reference voltage becomes equal to the signal voltage, and the digitizer is then in condition to be read out.

FIGURE 2 illustrates a modification of the comparatormodulator of FIGURE lb. In this modification, the lead network 5 2 is not present, although a resistor 56a corresponding to the lead network resistor 56' remains as a part of the summing network 54. The general function of the lea-d network in this case is assumed by a variable frequency, voltage controlled oscillator 72a. The frequency of oscillator 72a may be controlled by a reactance tube circuit responsive to the sum of the signal and reference voltages appearing on line 68. The oscillator is preferably adjusted so as to maintain its maximum frequency until the reference voltage is within ZS-SO-counts of balancing the signal voltage, whereupon the further decrease in the negative sum of the voltages on line 68 will gradually reduce the oscillator frequency to a minimum.

The voltage controlled oscillator 72a may of course be employed drive a chopper modulator-demodulator as in FIGURE lb. The use of a mechanical chopper, however, is generally limited to frequencies below two kilocycles, whereas the glow discharge or beam distributor tube counters are adapted to much higher counting rates. Accordingly FIGURE 2 illustrates the use of a diode ring type modulator-demodulator 70a as a comparator adapted to these higher counting rates.

In the above description, it has been assumed that the counter reference voltage generator produces a linear step-voltage function with the resistor values given in the accompanying table. It will be apparent that this is not strictly the case. It would be an obvious conclusion that true linearity could be achieved by slight alterations of the potentiometer resistor values from the values given. However, such conclusion does not take into account the fact that the impedance of line 6-4 to ground does not remain constant. Referring again to FIGURES 1a and 1b, it is seen that each potentiometer resistor as at 122, in series with its corresponding cathode resistor as at 102, provides a path from line 64 to ground, in parallel with the resistors in the summing network. It is also seen that the loading effect of such a parallel re sistance path is eliminated when the glow discharge is formed on the connected cathode. The resulting change in the impedance of line 64 to ground not only affects the reference voltage across the summing network, it also alfects the superimposed negative signal voltage as it is presented to the comparator-modulator.

The non-linearities produced by these impedance 7 changes cause nov practical difficulties in the units 46 and tens 48 section of the counter generator, since the potentiometer resistors in any combinationcan still present a relatively large impedance in comparison with the resistors in the lead network and/or summing network. In

this case, good linearity for all practical purposes also results from the fact that any firing cathode has connected thereto a potentiometer resistor whose value is very high in comparison with the impedance to ground presented by the lead network and summing resistors.

The linearity of the system is significantly improved by the design of the summing network 54 illustrated, wherein the signal voltage is divided by a fixed ratio voltage divider 66 and 62 and the signals from the divider and the counter-reference source are summed through resistors 60 and 56 (the lead network resistor) to the chopper contact 74b, thus providing a substantially constant output impedance for both the divider and the reference.

In digitizing to three figures or more, however, the values of the potentiometer resistors in the higher order decades may need to be reduced to such an extent that one or a combination of additional expedients must be employed to minimize the changes in the impedance of 'line 64 to ground.

FIGURE 3 illustrates one such expedient wherein the hundreds unit 50 employs crystal diodes as at 200 conmeeting each cathode as at 202 to the associated potentiometer resistor as at 204. The diode presents a high impedance when the potential of line 64 is higher than that of the cathode, as is the case when the cathode is not firing, and a low impedance when the glow discharge is formed on the cathode.

Another obvious expedient equivalent to that shown in FIGURE 3 would consist in connecting each cathode to its associated potentiometer resistor through an individual buffer amplifier.

As shown in FIGURE 4, the impedance of line 64 to ground 12b can be made initially very low in comparison with the parallel combination of potentiometer and cathode resistors by utilizing a small resistance 206 connecting line 64 to ground. Since this also results in a low voltage output from the counter reference generator, the generator output is amplified by a conventional operational amplifier 208'. The output of the operational amplifier is then summed with the signal voltage 011 line 10a in a conventional summing network 54a.

FIGURE 5 illustrates the circuitry of a .digital follower system in accordance with the invention. This system employs bi-directional counter tubes as at 220 and 222 of the decade distributor variety such as Dekatron type 6476 or type 6910 which are manufactured by Sylvania Electric Products, Inc, New York, N.Y. These tubes include ten cathodes designated zero to nine and two different beam. switching electrodes as at 224 and 226 indicated within the envelope of tube 220. When the add electrode 224 is pulsed, the beam switches from one cathode to the next in ascending order; when the subtract electrode 226 is pulsed, the beam switches from one cathode to the next in descending order.

The circuitry associated with the cathodes of the Dekatron tubes 220 and 222 may be identical with that described herein-above, as may be the summing network 54, modulator switch 74a-74c and modulator amplifier 78. Connected to the output of amplifier 78 is a transformer 23% having two secondary windings 23th: and 23% having a common connection to ground and to the vibrating contact 74d of the chopper 74. The opposite end of winding 23% is connected through a rectifier 23 2, a current limiting resistor 234 and a load resistor 236 to ground. The opposite end of winding 23Gb is similarly connected through a diode 238 and resistors 24d and 242 to ground. The transformer windings are so connected that the respective output voltages appear in phase, and positive-going half cycles thereof are transmitted to the respective resistor networks by the diodes to provide positive pulses across the load resistors 236 and 242. The input of an add pulse amplifier 244 is connected across resistor 242 and similarly the input of a subtract.

pulse amplifier 246 is connected, across resistor 236.

In operation, when the negative signal input on line 10a exceeds the positive voltage on line 64 from the counter reference voltage generator, the positive-going pulses transmitted by diodes 232 and 238 appear when the vibrating contact 74d of the chopper is connected to the contact 74c. Hence the load resistor 236 is shortcircuited by the chopper contacts and there is no signal into the subtract pulse amplifier 246. However, since the other contact 74f of the chopper is open, pulses do appear across resistor 242, thus triggering the add pulse amplifier 244 which feeds pulses via line 250 to the add switching electrode 224 of the units counter tube 220. When the positive signal from the counter reference voltage generator is larger than the negative signal input, the positive-going pulses transmitted by diodes 232 and 238 appear when load resistor 242 is short-circuited by chopper contacts 74d and 74f, and the add pulse amplifier 24-4 therefore receives no input pulses. At this time, however, the pulses across resistor 236 will trigger the subtract pulse amplifier 246, which sends amplified pulses over line 252 to the subtract switching electrode 226 of the units counter tube 220.

When the units counter tube 220 is counting in the adding or subtracting mode, the tens counter tube 222 and higher order counters (not shown) must also operate in the same mode. Accordingly a dual-output directionresponsive pulse amplifier 254 is used to cascade the units counter tube with the tens counter tube 222. The pulse amplifier 254 is shown to include a twin triode tube 256 whose cathodes 258 are commonly routed to ground 12b through a switching transistor 260 and a Zener diode 262; the cathodes being connected to the collector 260w, and the emitter 26Gb connected to the Zener diode.

. The grids 264a and 26% of tube 256 are respectively couv prising a resistor 288 and a capacitor 290.

are returned to ground through resistors 278 and 280, and their rest potentials are equalized by a resistor 282 interconnecting the grids. Line 274 from the nine cathode 266 and line 276 from the zero cathode 268 of the counter tube 220 are respectively connected through diodes 284 and 286 to an integrating circuit com- One plate of the capacitor 290 is grounded, and the other plate is connected to the base 2600 of the switching transistor 260*.

The direction-responsive pulse amplifier 254 determines when an add pulse, a subtract pulse, or no signal should be sent to the tens counter tube 222 in accordance with the mode of transfer of the beam discharge to and from the zero and nine" cathodes of the units counter tube 220. When the beam discharge transfers from the nine cathode 266 to the zero cathode 268, it indicates that the units counter capacity has been exceeded in the adding mode, and therefore an add pulse should be sent to the tens counter tube 222. When the beam discharge transfers from the zero cathode to the nine cathode, it indicates that the units counter capacity has been depleted in the subtracting mode, and therefore a subtract pulse should be sent to the tens counter. However, when in the adding mode the beam is normally transferred to the nine cathode from the eight cathode, and in the subtracting mode the beam is normally transferred to the zero cathode from the one cathode; in which cases neither add nor subtract pulses should be sent to the tens counter.

Pulse amplifier 254 functions to achieve these results, as follows. Assume that the beam in the units counter tube 220 is formed on one of the cathodes one to eight. Lines 274 and 276 connected respectively to the nine and zero cathodes are at ground potential. Now if the tube 220 is counting in the adding mode the beam will advance along the cathodes in ascending order until it is formed on the nine cathode 266, whose potential will rise. The resulting pulse transmitted to the control grid 264a through line 274 and capacitor 270 will have no effect, however, since the transistor 260 is in a nonconducting state so that the cathode 258b is effectively open-circuited. The positive voltage on the grid 264a quickly decays as coupling capacitor 270 discharges through resistor network 278482. The positive voltage on the nine cathode 266 and line 274 now charges the integrating capacitor 290 through diode 284 and resistor 288. The capacitor 290 supplies increasing current to transistor base 2600 and the collector 260a circuit gradually becomes conducting. The cathodes 258a and 258b of amplifier tube 256 are then biased by the voltage across the Zener diode 262. If an add pulse is now received on switching electrode 224 of counter tube 220, causing the beam discharge to switch from the nine cathode 266 to the zero cathode 268 the potential rise of the zero cathode and line 276 will cause a positive pulse to be transmitted through coupling capacitor 272 to the grid 264b of the amplifier tube 256. The associated plate 292 is connected by line 294 to the add switching electrode 296 of the tens counter tube 222 so that the resulting fall of the plate potential comprises an add pulse which advances the reading of the tens counter by one digit. If the beam in the units counter tube is now transferred from the zero cathode 268 to the one cathode so that both lines 274 and 276 are near ground potential, the integrating capacitor 290 will discharge through the base 2600 circuit of the transistor 260 and the Zener diode 262 until the transistor 260 again becomes non-conducting.

A similar action obtains when the counter tube 220 is operating in the subtracting mode. Thus if the beam therein is transferred from the one cathode to the zero cathode, the resulting pulse on amplifier grid 2641; has no effect. The subsequent charging of integrating capacitor 290 through resistor 288 and diode 286 then enables the cathode circuit of tube 256 as above described. When the beam is thereafter transferred from the zero to the nine cathode, the resulting pulse on grid 26401 results in an amplified pulse on the plate 298', which is connected to the subtract switching electrode 300 of the tens counter tube 222 through line 302.

(lonnected to the tens counter tube 222 there is shown a further direction-responsive dual-output pulse amplifier 304 which is identical with amplifier 254 and which provides add and subtract pulses to a hundreds digit counter (not shown). in this manner there is provided a digital follower capable of very rapidly digitizing successive values of a variable analog signal online 10a.

FEGURE 6 illustrates how the present invention may be adapted for use as a dividing element in a digital computer for directly digitizing the ratio of two analog voltages. In this case the first signal voltage is connected to input terminals 10 and 12 as before. The second signal voltage may be connected to input terminals 16 and 12a in place of the constant reference voltage source (battery) 18 shown in FiGURE la. As is obvious from the fore going description, the digitizer circuits represented generally by the box 310 require that the input voltage on line 16a, to which the counter cathode voltages are clamped, be several orders of magnitude larger than the input voltage on line 10a. Accordingly, it will usually be necessary to amplify signal No. 2 by means of a conventional operational amplifier 312, preferably having input 314 and feedback 316 resistors whose impedance ratio may be selected to: provide the necessary closed loop gain of the amplifier.

While the invention has been illustrated and described in connection with only a few selected and specific embodiments wherein it is apparent that the various objects of the invention have been accomplished, such specific embodiments are to be considered illustrative only and not restrictive, since a great many changes and modifications to the disclosed apparatus can be made without departing from the scope of the invention as is set forth in the appended claims;

What is claimed is:

1. In a digitizer for an input signal voltage, a pulse counter having an array of electrical discharge paths arranged in the orders of a digital code, means energized by the flow of electrical current in said discharge paths for generating a quantized voltage functional of the distribution of discrete currents in the denominational portions of said array; a voltage divider for said quantized voltage including an output impedance for providing a reference voltage consisting of a portion of said quantized voltage, and a lead network for adding a time derivative of said reference voltage to the value thereof; means generating time-spaced pulses for switching components of said current flow through said counter discharge paths in accordance with said digital code, and means responsive to said input signal voltage, said reference voltage and said time derivative thereof for disabling said pulse generating means when the sum .of said reference voltage and said time derivative voltage exceeds said signal voltage.

2. Electronic apparatus for digitizing a function of a first input voltage relative to a second input voltage, comprising a counter reference voltage generator including a plurality of electrical discharge paths in parallel relation, each of said paths being characterized by a conductive state and an alternative substantially nonconductive state, a source of supply voltage connected to said discharge paths, means triggered by electrical pulses for switching said discharge paths successively into said conductive state, a plurality of load impedances, each of said load impedances connected in circuit with one of said discharge path is in said conductive state, a plurality of clamping means, each of said clamping means being connected in circuit with one of said load impedances and coupled to said second input voltage for clamping said potential at a value proportional to said second input voltage, a plurality of potentiometer impedances having a series of mutually different values determined by said function, each of said potentiometer impedances being coupled to one of said load impedances so as to be energized by said potential thereon, and a common output impedance forming a voltage divider with each of said potentiometer impedances for providing said counter reference voltage; means responsive to a difference between said counter reference voltage and said first input voltage for generating electrical pulses, and means connecting said electrical pulses to said discharge path switching means.

3. Apparatus as in claim 2 including a standard source of constant reference potential for providing said second input voltage.

4. In a digitizer for an input signal voltage, a counter having a pulse input and a register including an array of electrical discharge paths sequentially energized in a denominational pattern to encode the total of a number of pulses applied to said pulse input, means energized by the flow of currents in said discharge paths for generating a step voltage reference signal functional of said total in said register, a variable frequency oscillator having frequency adjusting means, a pulsegenerator driven by said oscillator for providing time-spaced pulses to said counter input at the frequency of said oscillator, means for adding said reference signal and said input signal in mutual opposition to obtain a difference voltage, and means controlled by said difference voltage for actuating said oscillator frequency adjusting means.

5. In a digitizer for a first input signal voltage, a

counter reference voltage generator comprising a plurality of cold-cathode glow-discharge distributor tubes, each of said tubes including a common anode, a plurality of cathodes, and a switching electrode for sequentially transferring said glow discharge from one of said cathodes to the next, a supply voltage connected to said distributor tubes, a second input voltage, a plurality of load impedances, each of said load impedances being connected in circuit with one of said cathodes for generating a potential when said glow discharge is formed on said one cathode, a plurality of clamping means, each of said clamping means being connected in circuit with portional to said second input voltage, a plurality of potentiometer impedances having a series of mutually different values determined by a desired function of said first input voltage, each of said potentiometer impedances being coupled to one of said load impedances so as to be energized by said potential thereon and a .common output impedance forming a voltage divider with each of said potentiometer impedances for providing said counter reference voltage; a pulse generator energized only by a difference between said counter reference voltage and said first input voltage, said pulse generator having an output connected to said switching electrode of a firsttone of said distributor tubes, and a pulse amplifier connecting one cathode of said first dis- .tributor tube to the switching electrode of a second one of said distributor tubes.

6. The combination of claim 5 wherein said pulse generator comprises a summing network for comparing said reference voltage and said first'input voltage to provide-a difference voltage, a source of alternating voltage, a modulator driven by said alternating voltage for transforming said difference voltage into a modulated voltage having a first phase relation to said alternating voltage when said first input voltage exceeds said reference voltage and a second phase relation when said reference voltage exceeds said first input voltage, and a pulse amplifier triggered only by said modulated voltage having said first phase relation for providing pulses at said pulse generator output.

7. The combination of claim.6 wherein said voltage divider includes a resistance-capacitance lead network connecting said potentiometer impedances to said common output impedance.

8. The combination of claim 6 wherein said source of alternating voltage comprises a variable frequency oscillator, and means for controlling the frequency of said oscillator in accordance with the amplitude of said difference voltage.

References Cited in the file of this patent UNITED STATES PATENTS 2,496,912 Grosdoif Feb. 7, 1950 2,836,356 Forrest et a1. May 27, 1958 2,845,528 Brook July 29, 1958 2,845,597 Perkins July 29, 1958 2,870,436 Kuder Jan. 20, 1959 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,026,031? March 20, 1962 David L. Spooner It is hereby certified that error appears in the above numbered patant requiring correction and that the said Letters Patent should read as corrected below.

In the headingto the printed specification, lines 2 and 3, in the title of invention, for FRACTIONS read FUNCTIONS column 10, line' 70, after "said", first occurrence, insert discharge paths for generating a potential when said one Signed and sealed this 3rd day of July 1962.

(SEAL) Attest:

ERNEST w. SWIDER DAVID LADD Attesting Officer Commissioner of Patents 

